The present application relates to the medical imaging arts. It particularly relates to combined magnetic resonance (MR) and positron emission tomography (PET) imaging systems, and is described with particular reference thereto. The following relates more generally to imaging systems that combine the MR imaging modality with a modality employing energized particles, such as the aforementioned PET modality, single photon emission computed tomography (SPECT) modality, transmission computed tomography (CT) modality, a radiation therapy modality, or so forth.
In a hybrid imaging system, two or more medical imaging modalities are integrated into the same facility or room, or even into the same gantry. Hybrid imaging systems enable medical personnel to combine the advantages of the constituent modalities to acquire more useful information about the patient. Hybrid imaging systems also make it easier to spatially and temporally register images from the constituent modalities as compared with acquiring such images by discrete, separate imaging systems. Separate imaging systems have a longer lag time between studies, and make it difficult to minimally disturb the patient between studies.
The advantages of hybrid imaging systems have been realized commercially. For example, the Precedence SPECT/CT system available from Philips Medical Systems, Eindhoven, The Netherlands provides a CT scanner and a gamma camera for SPECT imaging. The latter includes two radiation detector heads mounted on robotic arms offset from the CT gantry along the patient end of the system. An extended patient couch is used to allow for adequate axial movement of the patient. Thus, both CT and SPECT imaging capability are available with limited modifications to either the CT gantry or the spatially separated gamma camera. Similarly, the Gemini PET/CT system also available from Philips Medical Systems, Eindhoven, The Netherlands provides both PET and CT imaging modalities.
However, construction of a hybrid imaging system including a magnetic resonance (MR) scanner and a second modality imaging system employing high energy particles or photons (such as SPECT or PET) is challenging. In a typical magnetic resonance imaging facility, a magnetic resonance scanner is located in a specially designed radio frequency isolation space created by a surrounding Faraday cage-type radio frequency shield. The radio frequency isolation space protects the sensitive magnetic resonance detection system from extraneous radio frequency interference. Additionally, the radio frequency (RF) shield helps reduce radiofrequency emissions from the MR scanner's RF transmit coils to the environment external to the scanner room. Problematically, the electronics for radiation detectors used in PET scanners or other imaging systems that detect high energy particles or photons typically generate high levels of radio frequency interference. Conversely, the magnetic field that is produced by the magnetic resonance scanner distorts the response of the photon detectors used in the PET scanner. Consequently, when considering placement in the same room with close proximity, there is an inherent practical incompatibility between a magnetic resonance scanner and an imaging system that detects high energy particles or photons.
Cho et al, U.S. Published Application No. 2006/0052685, proposes overcoming this inherent incompatibility by disposing the PET scanner outside of the radio frequency isolation space containing the magnetic resonance scanner. Unfortunately, this approach vitiates many of the benefits of a hybrid MR/PET system. The patient must be transferred between the MR and PET systems through a shutter-type opening in a wall of the radio frequency isolation room containing the MR scanner. Medical personnel must move back and forth between the room containing the PET scanner and the radio frequency isolation room containing the MR scanner. The system of Cho et al. includes a long railway system for transferring the patient between the MR and PET scanners located in separate rooms. The patient may find such a long-distance transfer uncomfortable, and shifting or other movement of the patient during such a long transfer can introduce spatial registration errors in images acquired by the MR and PET. Moreover, difficulties can arise in transferring local coils used in magnetic resonance imaging across the long rail distance.
Another approach that has been proposed is to integrate the PET radiation detectors into the gantry of the magnetic resonance scanner. It has been suggested that by judicious positioning of the radiation detectors at null or low strength points of the magnetic field, the effect of stray magnetic fields on the PET radiation detectors can be reduced. However, such low magnetic field regions, even in the case of an active-shield MR magnet, occur generally outside of the MR gantry at a relatively large radius near the midplane, thus requiring PMTs (photomultiplier tubes) to be relatively displaced from the PET detector material with a relatively long light guide path there-between; this approach reduces overall PET detector efficiency. Also, this approach does not address the issue of radio frequency interference from the radiation detectors interfering with the magnetic resonance detection system. Additionally, the integrated PET radiation detectors occupy valuable bore space in the MR scanner. Still further, even with judicious positioning of the PET radiation detectors' PMT components at null points of the MR magnetic field, it can be expected that some stray magnetic fields from the MR system or from other sources may still cause problems for the PET imaging.
A variation on the integrated approach noted above, disclosed in Hammer, U.S. Pat. No. 4,939,464, is to integrate only the scintillators of the PET scanner into the magnetic resonance scanner. Scintillation light produced by radiation detection events is captured and transferred by fiber optics to remote optical detectors of the PET system. This approach reduces, but does not eliminate, MR bore space usage by PET components, and additionally introduces sensitivity issues in the PET system due to optical losses in the extensive fiber optical light coupling systems. Moreover, while arranging the light detection electronics remotely is beneficial, some types of scintillation crystals exhibit spontaneous radioactivity that can still produce substantial radio frequency interference.
Solid-state PET detectors that are insensitive to the strong magnet field of the MR system provide another alternative for bore-integration of PET with MR. However, it remains the case that for whole-body applications, valuable bore space must be traded-off and/or a high level of complex and costly integration of PET components with MR gradient, radiofrequency transmit/receive body coil, and bore covers must be realized in this alternative.
A disadvantage of existing hybrid approaches is that these approaches are not conducive to retrofitting an existing magnetic resonance scanner. The approach of Cho et al. requires availability of a PET scanner room suitably located adjacent to the radio frequency isolation room of the magnetic resonance scanner, and further requires cutting a passthrough into the separating wall and adding a complex and bulky railway system for coupling the PET and MR scanners located in separate rooms. Approaches that integrate the PET radiation detectors into the MR scanner bore similarly add complexity to the retrofitting process, and may be unworkable with some existing MR scanners, particularly for whole-body applications.
The illustrative example of a hybrid PET/MR system is one in which stray magnetic fields are expected to be particularly problematic for the PET imaging. However, PET imaging systems in other contexts can suffer from stray magnetic fields. For example, other instruments that employ magnetic fields such as radiation generators for radiation therapy systems, electron microscopes, and so forth can produce problematic stray magnetic fields. Indeed, even the earth's magnetic field can be a problematic stray magnetic field for sensitive PET imaging.